County Ag News

Late Blight in Central Wisconsin


Late blight so far has been detected in commercial potato fields in three Central Wisconsin counties. On June 23, a field in northern Adams County was found with late blight. The second field was in western Waushara County on July 7 and the third in southern Wood County on July 8. 

Vegetables and field crops can sustain significant yield reduction from fungal diseases or in the case of late blight, total crop loss, and need to be protected by the use of fungicides. 

Home gardeners growing potatoes and tomatoes also need to spray their plants every 5-7 days with a fungicide, not only to protect their plants but also to prevent their garden from becoming a source of inoculum for potatoes on area farms.

Late blight symptoms include large water-soaked, dark brown lesions with sporulation on foliage. Lesions quickly expand to blight entire leaflets, stems, and fruits. Late blight is a potentially destructive disease of potatoes caused by the fungal organism, Phytophthora infestans

This pathogen is referred to as a ‘water mold’ since it thrives under wet conditions. All potato plant parts can become infected by late blight, with leaf lesions beginning as pale green or olive green areas that quickly enlarge to become brown-black, water-soaked, and oily in appearance. 

Lesions on leaves can also produce pathogen sporulation, which looks like white-gray fuzzy growth. Stems can also exhibit dark brown to black lesions with sporulation. The time from first infection to lesion development and sporulation can be as fast as 7 days, depending upon the weather. 

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